This invention relates to an aqueous solution for electroless autocatalytic copper plating, containing a source of cupric ions, a source of hydroxylic ions, a reducing agent and a complexing agent in such a quantity as to render soluble the cupric ions in an alkaline medium.
The electroless autocatalytic copper plating baths, also called "chemical copper baths", are capable of depositing a layer of copper onto a catalytic support, without the aid of an outer source of electrons. Generally, these baths are constituted by aqueous solutions containing a copper salt, a copper complexing agent, a reducing agent and a regulator of the pH.
The copper may be used in the form of sulphate, halid, nitrate, fluoroborate, acetate or in the form of other inorganic or even organic salts. Generally, for economical reasons it is preferred to employ the copper in the form of pentahydrated sulphate.
The function of the complexing agent is to keep the copper in solution in the alkaline medium which is necessary for the deposition reaction. To this end it is possible to utilize Rochelle salt (sodium and potassium tartrate) or gluconic acid or gluconates, nitrilotriacetic acid or its alkaline salts, triethanolamine, or also complexing agents such as ethilene diamine tetracetic acid and its sodium salts, N-hydroxy-ethyl-ethylen-diamine triacetate, N,N,N.sup.1,N.sup.1,tetra-(2-hydroxypropyl)ethylenediamine etc.
The copper reducing agent normally used in the chemical copper baths is formed by formaldehyde or its derivatives or precursors, such as paraformaldehyde, trioxane or the like. Also utilized as reducing agents are the borohydrides of alkaline metals, such as sodium borohydride, or the boranes of the dimethylaminoborane type etc. To this end one may use also hypophosphites of alkaline metals.
The function of the regulator of the pH is to keep an optimal alkalinity degree for the oxidoreduction reaction which leads to the deposition of a layer of copper onto the catalytic support. Normally, the work is carried out with pH values between 10 and 14, and to keep these pH values sodium or potassium hydroxide is used.
Besides these fundamental components, the chemical copper baths also contain a series of products, present therein in small concentrations, such as stabilizers, wetting agents and so on.
The products which exert a stabilizing action and which are used in the common practice generally include mercury salts, cyanides of alkali metals, organic nitriles or compounds containing sulphur in bivalent form, such as 2-mercaptobenzotriazole, thiourea, inorganic sulphides such as the sulphides of alkali metals, or thiocyanates or dithionates of alkali metals. Generally these stabilizers are catalytic poisons, and many of them have the characteristic of greatly reducing the speed of deposition, or, in the extreme cases, even of completely locking the bath, i.e. rendering the same non autocatalytic.
In the use of these chemical copper baths to metallize metallic surfaces it is only necessary to carry out a thorough degreasing of the surfaces. However, when it is desired to metallize non metallic surfaces it is necessary for these surfaces to be rendered catalytic and hence receptive in respect of the subsequent deposition of chemical copper.
To this end, generally, a treatment is carried out whose function is to carefully degrease the surfaces and to condition them in order to promote, in the subsequent catalysis stage, the adhesion of a thin and uniform layer of a noble metal.
The successive operation includes bringing said surfaces into contact with the catalyst solution which generally is based on palladium chloride and stannous chloride in a solution acidic by presence of HCl or in saline solution with NaCl or LiCl. This solution contains a compound between palladium and tin, present in colloidal form. Thereafter, the acceleration stage is carried out, whose function is to prevent coarse catalyst particles, which do not adhere to the substrate to be metallized, to be transferred into the chemical copper bath. Furthermore, this bath has the function of increasing the palladium/tin ratio within the colloidal complex in order to increase its catalytic power.
Generally, this solution is a solution acidic by presence of fluoboric acid, perchloric acid or the like, or it is a solution alkaline by presence of NaOH or other substances.
The electroless deposition of copper is applied in the decorative field and in the electroforming technique, in which onto a support of plastics, rendered conductive by a deposit of chemical copper, there is electrodeposited a suitable thickness of metal which thus can assume shapes which cannot be obtained by any other working technique. The electroless copper deposition, however, finds its application above all in the electronic field, and in particular in the production of printed circuits with metallized holes.
To obtain a consistent thickness in an acceptable time it is necessary to use a bath having a high deposition speed, which generally operates at a high temperature or contains a considerable concentration of active components, such as the source of cupric ions and the reducing agent. In the practice, the conduction of baths having these operative characteristics proves to be very difficult because of the high unstability exhibited by these baths, with consequent copper plating of the bottom of the tanks and uncontrolled decompositions.
This invention finds its application in this field, and its aim is to allow an optimal stabilization of the bath without jeopardizing its operative characteristics of deposition speed and the physical characteristics such as gloss and ductility, of the deposit which is obtained.